Reproducing device for frequency modulated signals

ABSTRACT

A frequency modulated signal reproducing device includes a reproducing device for reproducing a frequency modulated signal from a recording medium; a pilot signal generating circuit for generating a pilot signal relative to a discontinued part of the signal reproduced by the reproducing device; a phase lock circuit which generates a phase locked signal which is phase locked relative to the reproduced signal; a demodulating circuit for frequency demodulating the phase locked signal; and a control circuit for controlling the response speed of the phase lock circuit on the basis of the pilot signal.

This is a division of application Ser. No. 615,785, filed May 31, 1984,U.S. Pat. No. 4,692,914.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a frequency modulated signal reproducingdevice, and more particularly to a device arranged to reproduce aninformation signal which is recorded with frequency modulation and hasno time correlativity.

2. Description of the Prior Art

In reproducing an information signal having time correlativity, such asa video signal or the like, when a discontinuity arises in the signaldue to a drop-out or the like, such a discontinuity has been, ingeneral, compensated for with some other part of the signal that iscorrelated with the discontinuity. However, in the event of occurrenceof such a discontinuity in an information signal reproduced from anaudio signal which has no time correlativity, the discontinuity cannotbe replaced with another part in the above-stated manner. Especiallyoccurrence of such a discontinuity in a reproduced frequency modulatedsignal greatly disturbs the information as it results in noise of largeamplitude in the information signal obtained after frequencydemodulation.

During recent years, there have appeared two-head helical scanning typevideo recorders (hereinafter will be called VTR) which are arranged toreproduce an audio signal from a recording tape containing a multiplesignal consisting of a frequency modulation recorded audio signal and arecorded video signal. In the VTR of this type, the frequency modulatedaudio signal becomes discontinuous between the output of one head andthat of the other. Then, this discontinuity brings about some slighttime difference. The reproduced frequency modulated audio signal isphase modulated at the time of switch-over from one head to the other.As a result of this, a pulse noise of large amplitude is included in afrequency demodulated output. With the demodulated output allowed topass through a low-pass filter (hereinafter will be called LPF), theretakes place a transient distortion. This brings about a noise whichlasts over a certain given period of time after the above-stated headswitch-over. In addition to this, assuming that the heads are arrangedto make 60 turns per sec., the noise arises at that frequency.Therefore, the noise becomes quite disagreeable to the ear as it teemswith such 60 Hz components, for example, and their higher harmonics.

To solve this problem, therefore, in case that an information signalhaving no time correlativity, such as an audio signal, is to bereproduced and frequency demodulated, there have been employed variousnoise eliminating methods. In one such method, the audio signal part foreach period during which the reproduced signal discontinues due tooccurrence of a drop-out or switch-over from one reproducing track toanother, is arranged to be muted. In another method, the audio signallevel immediately before the discontinued period is held over apredetermined period of time. FIG. 1 of the accompanying drawings showsa circuit arrangement of the conventional frequency modulated audiosignal reproducing system provided with a drop-out compensatingfunction. Referring to FIG. 1, the conventional system comprises aterminal 1 which is arranged to receive a frequency modulated audiosignal reproduced by reproducing means, such as a head or the like; aband-pass filter 2 (hereinafter called BPF); a frequency demodulator 3;a low-pass filter (LPF) 4; a prior-value holding circuit 5; a drop-outdetection circuit 6; a monostable multivibrator 7; an OR circuit 8; andan audio signal output terminal 9.

FIG. 2 is a waveform chart showing the output waveforms of the points(a)-(f) in FIG. 1. The conventional system operates in the followingmanner:

The reproduced frequency modulated audio signal, which is supplied tothe input terminal 1 passes through the BPF 2 and is demodulated by thefrequency demodulator 3. The demodulated signal is converted to an audiosignal through the LPF 4. If the reproduced frequency modulated signalcoming to the frequency demodulator 3 discontinues due to a drop-out,the waveform of the audio signal produced from the LPF 4 becomes asshown in FIG. 2(b). Further, the waveform of a drop-out detection signalobtained from the drop-out detection circuit 6 becomes as shown in FIG.2(c).

The sound noise, which is produced in the event of a drop-out as shownin FIG. 2(b), still remains even after the generation of the drop-outdetection signal shown in FIG. 2(c). This is attributable to thetransient characteristic of the LPF 4. In view of this, therefore, adrop-out detection pulse is supplied to the monostable multivibrator 7to cause it to produce a pulse as shown in FIG. 2(d) during a periodcorresponding to the transient distortion generating period of the LPF4. Then, a waveform which is shown in FIG. 2(e) is obtained bysubjecting the drop-out detection pulse and the output pulse of themonostable multivibrator 7 to a logical sum obtaining process performedby the OR circuit 8. Then, the prior-value holding circuit 6 is operatedby the output pulse of the OR circuit 8 to obtain the waveform of thereproduced audio signal as shown in FIG. 2(f). However, the reproducedaudio signal thus obtained greatly differs from the original recordedaudio signal and thus presents a problem in terms of fidelity because ofthe prolongation of the prior-value holding period resulting from thetransient characteristic.

FIG. 3 shows a circuit arrangement of another prior art frequencymodulated audio signal reproducing system which is provided with afunction of removing the above-stated noise produced during switch-overof the heads of a VTR. The system comprises input terminals 11 and 12,arranged to receive a frequency modulated audio signal reproduced bydifferent heads; a switching circuit 13, arranged to selectively producethe signal received by the terminal 11 or 12 according to a headswitch-over pulse which will be described later herein; a BPF 14; afrequency demodulator 15; an LPF 16; a prior-value holding circuit 17;an input terminal 18, which is arranged to receive the head switch-overpulse; a hold pulse generation circuit 19; and an audio signal outputterminal 20. FIG. 4 is a waveform chart or a timing chart showing thewaveform at the points (a)-(g) of FIG. 3. The operation of this priorart system will be described below with reference to the waveform chartof FIG. 4.

The frequency modulated audio signal, which is supplied to the inputterminal 11 of FIG. 3 having a waveform as shown in FIG. 4(a), and thefrequency modulated audio signal, which is supplied to the inputterminal 12 having a waveform as shown in FIG. 4(b), are put together bythe switching circuit 13 to obtain a continuous frequency modulatedaudio signal as shown in FIG. 4(d). Then, this audio signal is suppliedto the frequency demodulator 15. However, if there is a time differencebetween the frequency modulated audio signals supplied to the terminals11 and 12, phase modulation is caused by a phase difference resultingfrom the time difference existing at the time of signal switch-overeffected by the head switch-over pulse supplied to the terminal 18.Therefore, the output of the frequency demodulator 15 includes a pulsenoise due to the phase modulation which takes place at the time of headswitch-over. Then, due to the transient characteristic of the LPF 16, anoise of large amplitude appears in the output waveform of the LPF 16 asshown in FIG. 4(e). Meanwhile, the head switch-over pulse supplied tothe head switch-over pulse input terminal 18 is supplied also to thehold pulse generation circuit 19, which produces a hold pulse as shownin FIG. 4(f). The prior-value holding circuit 16 is then operated withthis hold pulse. As a result of this, the audio signal output terminal20 produces a signal of a waveform which is as shown in FIG. 4(g).However, due to the prior-value holding period which is prolonged by thetransient characteristic of the LPF, the reproduced audio signal thusobtained deviates from the original recorded audio signal. Therefore, itbecomes impossible to reproduce the original sound with fidelitythereto.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a frequencymodulated signal reproducing device which is capable of reproducing withfidelity the original signal to eliminate the shortcoming of the priorart described in the foregoing.

It is another object of the invention to provide a frequency modulatedsignal reproducing device which is capable of removing noise of largeamplitude produced by discontinuation of a reproduced frequencymodulated signal.

It is a further object of the invention to provide a frequency modulatedsignal reproducing device capable of preventing occurrence of a periodicnoise due to discontinuation of a reproduced frequency modulated signalresulting from switch-over from one reproduction track to another.

To attain these objects, a preferred embodiment of the inventioncomprises (a) reproducing means for reproducing a frequency modulatedsignal from a recording medium; (b) pilot signal generating means forgenerating a pilot signal relative to a discontinued part of the signalreproduced by the reproducing means; (c) phase lock means whichgenerates a phase locked signal which is phase locked relative to thereproduced signal; (d) demodulating means for frequency demodulating thephase locked signal; and (e) control means for controlling the responsespeed of the phase lock means on the basis of the pilot signal.

These and further objects and features of the invention will becomeapparent from the following detailed description of preferredembodiments thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an example of a conventional frequencymodulated signal reproducing device.

FIG. 2, consisting of (a)-(f), is a timing chart showing the waveform ofthe output at each point in the conventional device shown in FIG. 1.

FIG. 3 is a block diagram showing another example of a conventionalfrequency modulated signal reproducing device.

FIG. 4, consisting of (a)-(g), is a timing chart showing the waveform ofthe output at each point in the device shown in FIG. 3.

FIG. 5 is a block diagram showing the arrangement of essential parts ofa frequency modulated signal reproducing device arranged according tothe invention as an embodiment thereof.

FIG. 6, consisting of (a)-(d), is a timing chart showing the waveform ofthe output at each point in the embodiment shown in FIG. 5.

FIG. 7 is an illustration of relationship between an input signal and anoutput signal of the frequency demodulator of the embodiment.

FIG. 8 is a block diagram showing the arrangement of essential parts ofa frequency modulated signal reproducing device arranged as anotherembodiment of the invention.

FIG. 9 consisting of (a)-(e) is a timing chart showing the outputwaveform at each point in the device of FIG. 8.

FIG. 10 is a block diagram showing the arrangement of essential parts ofa frequency modulated signal reproducing device arranged as a furtherembodiment of the invention.

FIG. 11, consisting of (a)-(i), is a timing chart showing the outputwaveform at each point in FIG. 10.

FIG. 12 is a block diagram showing the circuit arrangement of thereproduction system of a VTR arranged according to the invention as anembodiment thereof.

FIG. 13 is a block diagram showing the arrangement of essential parts ofa frequency modulated signal reproducing device arranged as anotherembodiment.

FIG. 14, consisting of (a)-(g), is a timing chart showing the outputwaveform at each point in FIG. 13.

FIG. 15 is a block diagram showing the essential parts of a frequencymodulated signal reproducing device arranged as a still furtherembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the accompanying drawings, preferred embodiments of theinvention will be described below for a more complete understanding:

FIG. 5 shows the reproducing circuit arrangement of a frequencymodulated signal reproducing device as an embodiment of the invention.In FIG. 5, the components which are similar to those of FIG. 1 areindicated by the same reference numerals as those used in FIG. 1. Theembodiment includes a phase comparison circuit 30; a voltage controlledoscillator 31 (hereinafter called VCO); an LPF 32; and a time constantvarying circuit 33. FIG. 6 is a waveform chart showing the outputwaveform at each of the points (a)-(d) shown in FIG. 5. The operation ofthe embodiment is as follows:

Referring to FIG. 5, a frequency modulated audio signal reproduced by areproducing head 10 is supplied, via a BPF 2, to the phase comparisoncircuit 30. A phase lock loop (hereinafter called PLL) is formed by thephase comparison circuit 30, the VCO 31 and the LPF 32. A signalproduced from the VCO 31 is thus phase locked relative to the reproducedfrequency modulated audio signal supplied to the phase comparisoncircuit 30. The output of the VCO 31 is supplied to a frequencydemodulator 3. An audio signal which is demodulated by the frequencydemodulator 3 is supplied to the audio signal output terminal 9 via anLPF 4.

When the reproduced frequency modulated signal, which is being suppliedto the phase comparison circuit 30, comes to discontinue due to adrop-out, a drop-out detection circuit 6 produces a drop-out detectionsignal as shown in FIG. 6(c). During the period of generation of thissignal, the time constant varying circuit 33 increases the time constantof the LPF 32 included in the PLL in such a way as to delay the responseof the PLL. Therefore, in the event of occurrence of a drop-out, theresponse of the PLL causes the frequency of the signal produced from theVCO 31 to slowly change toward the free-running frequency of the PLL.With the free-running frequency set at the carrier center frequency ofthe recorded frequency modulated audio signal, the frequency of thesignal which is produced from the VCO 31 and supplied to the frequencydemodulator 3, as shown in FIG. 6(b), gradually becomes closer to thecarrier center frequency. Upon termination of the drop-out period, theresponse of the PLL becomes sufficiently faster to be able to follow thereproduced frequency modulated audio signal. Thus, the signal producedfrom the VCO 31 is phase locked to the input signal. The frequencydemodulator 3 and the LPF 4 then give a reproduced audio signal of awaveform as shown in FIG. 6(d).

In other words, the frequency demodulator 3 normally receives afrequency modulated signal which is phase locked relative to thereproduced frequency modulated audio signal. Then, in the event of adrop-out, the frequency demodulator 3 receives a signal of frequencywhich shifts from the frequency immediately before the occurrence of thedrop-out to the free-running frequency which is set at the carriercenter frequency of the frequency modulated audio signal. The frequencydemodulator 3 thus receives an input signal even during the drop-outperiod. This arrangement effectively prevents any transient distortiondue to the LPF 4, as such a noise that would be produced by supply of asignal under no signal condition is thus prevented from being produced.

FIG. 7 shows the waveform of a signal reproduced through the frequencydemodulator 3 with the phase of a signal supplied to the demodulator 3changed. Assuming that the degree of change in phase is Δφ and achanging time Δt, an instantaneous frequency change Δf within thechanging time Δt is expressed as:

    Δf=d(φ)/dt

The instantaneous frequency change Δf is detected by the frequencydemodulator 3 and is produced as a change in signal through the LPF 32.The signal change thus produced becomes a sound noise.

FIG. 8 is a circuit diagram showing the circuit arrangement of essentialparts of a frequency modulated signal reproducing device arranged asanother embodiment of the invention. The components similar to those ofFIG. 3 are indicated by the same reference numerals as those used inFIG. 3. This embodiment includes a phase comparison circuit 41; a VCO42; an LPF 43; a time constant varying pulse generation circuit 44; atime constant varying circuit 45; the phase comparison circuit 41; theVCO 42 and an LPF 43 forming a PLL. FIG. 9 is a timing chart showing theoutput waveform at the points (a)-(e) shown in FIG. 8. Referring now toFIG. 9, the circuit arrangement of FIG. 8 operates as follows: Frequencymodulated audio signals reproduced, respectively, by reproducing heads11' and 12' are combined together by a switch-over circuit 13 into afrequency modulated audio signal as shown in FIG. 9(a). The combinedaudio signal is supplied via a BPF 14 to the phase comparison circuit41. A signal produced from the VCO 42 is phase locked relative to thereproduced frequency modulated audio signal supplied to the phasecomparison circuit 41. A signal produced from the VCO 42 is supplied toa frequency demodulator 15. Then, the audio signal, which thus hasundegone a detection process, is supplied, via an LPF 16, to an audiosignal output terminal 20.

Meanwhile, to a head switch-over pulse input terminal 18 is supplied asignal of a waveform as shown in FIG. 9(b). The time constant varyingpulse generation circuit 44 then produces a signal of a waveform asshown in FIG. 9(c). In response to this, the time constant varyingcircuit 45 operates in such a way as to make the response time of thePLL later than the head switching time by a predetermined length oftime. Then, the phase difference of the signal produced from the VCO 42shows a characteristic as shown in FIG. 9(d). The signal detected by thefrequency demodulator 15 is supplied via the LPF 16 to the audio signaloutput terminal 20. The waveform of this signal is as shown in FIG.9(e).

With the embodiment arranged to have the circuit arrangement asdescribed above, the frequency modulated signals combined by theswitch-over circuit 13 are converted into the output signal of the VCO42 which is phase locked by the PLL. The output of the VCO 42 is FMdetected. With the response of the above-stated PLL arranged to be laterthe head switching time by a predetermined length of time, generation ofa noise of large amplitude in the reproduced frequency modulated audiosignal can be suppressed and the noises due to the head switching cycleand the contingent higher harmonic component also can be suppressed.

FIG. 10 shows a drop-out compensating circuit for a reproduced frequencymodulated audio signal of a reproducing device which is arranged asanother embodiment of the invention. In FIG. 10, the components of thedevice similar to those of FIG. 1 are indicated by the same referencenumerals as those used in FIG. 1. This embodiment includes a delaycircuit 50; an LPF 51 which has a cut-off frequency arranged to besuitably lower than that of another LPF 4; a change-over switch 52; andanother delay circuit 53. The delay times of both the delay circuits 50and 53 are arranged according to the cut-off frequency of the LPF 51.The arrangement is such that the time delay due to passing of the signalthrough the LPF 51 can be compensated for beforehand.

FIG. 11 is a timing chart showing the waveform in outputs at points(a)-(i) of FIG. 10. Referring to FIG. 11, the embodiment shown in FIG.10 operates as follows: A reproduced frequency modulated audio signal,which has been reproduced by a reproducing head 10, is allowed to passthrough a limiter 29 to have an amplitude varied portion thereof removedthereby, as shown in FIG. 11(a). The signal thus processed is suppliedto a frequency demodulator 3 and a drop-out detection circuit 6. In theevent of occurrence of a drop-out in the frequency modulated audiosignal, there appears noise of large amplitude and a transient noise inthe output of the LPF 4 which is connected to a stage subsequent to thedemodulator 3. These noises are as shown in FIG. 11(b). The output ofthe LPF 4 is supplied to the delay circuit 50 and the LPF 51. Thewaveforms of their outputs are as shown in FIG. 11(d) and FIG. 11(e).The cut-off frequency of the LPF 51 is lower than that of the LPF 4 andis sufficiently low for removing the noise of large amplitude and thetransient noise almost completely. The delay circuit 50 has a delay timewhich is equal to the length of time delay τ1 caused by the LPF 51. Theoutput of the delay circuit 50 is supplied to the terminal A of thechange-over switch 52 and that of the LPF 51 to the other terminal B ofthe switch 52.

When a drop-out is detected by the drop-out detection circuit 6, thecircuit 6 produces a pulse signal which remains at a high level duringthe period of the drop-out as shown in FIG. 11(c). The output of thedrop-out detection circuit 6 is delayed by the delay circuit 53, whichalso has the above-stated delay time τ1 as shown in FIG. 11(f). Thedelayed drop-out detection pulse signal is supplied to an OR gate 8 anda monostable multivibrator 7. The monostable multivibrator 7 triggersits output terminal at the rise of the input to produce thereby a pulsesignal which remains at a high level over a length of time τ2,corresponding to the period during which the transient noise isproduced, as shown in FIG. 11(g). As a result of that, the OR gate 8produces a pulse signal which is obtained by adding up these pulsesignals, as shown in FIG. 11(h). The switch 52 is connected to theterminal B thereof when the output level of the OR gate 8 is high and tothe other terminal A when the output level of the OR gate is low. Duringthe drop-out period of the reproduced audio signal and the transientnoise generating period shown in FIG. 11(d), therefore, the lowfrequency signal which is obtained through the LPF 51 is substituted toobtain a smooth correction signal as shown in FIG. 11(i).

With the reproducing device arranged as described above, in the event ofoccurrence of a drop-out in the reproduced frequency modulated audiosignal, the noise of large amplitude and the transient noise containedin the output of the frequency demodulator 3 is replaced with areproduced audio signal of low frequency which is free from thesenoises. Therefore, PG,18 compared with the holding method of the priorart, a smoother drop-out correction signal can be obtained.

FIG. 12 shows a reproduction circuit system of a VTR as an embodiment ofthe invention. In the case of FIG. 12, a mixed signal consisting of acolor video signal and an audio signal, is recorded with two heads ontoa magnetic tape adjacently forming recording tracks one after another onthe tape and then the recorded signal is reproduced. The invention isapplied to the reproduction system of the VTR. In mixing these signals,the VTR employs a known method in which the color video signal has itsluminance signal frequency modulated within a relatively high band; itschrominance signal frequency modulated within a low band; while theaudio signal is frequency modulated within an intermediate band. In FIG.12, the components similar to those shown in FIG. 10 are indicated bythe same reference numerals as those used in FIG. 10. The VTR isprovided with rotary heads 120 and 121 which are arranged on a cylinder(not shown) with their phases differentiated 180 degrees from eachother; pre-amplifiers 122 and 123; a head change-over switch 124; achrominance signal separating LPF 124; a luminance signal separatinghigh-pass filter 126 (hereinafter a high-pass filter will be calledHPF); an audio signal separating BPF 127; a chrominance signalprocessing system circuit 128; a luminance signal processing systemcircuit 129; a mixer 130; a reproduction amplifier 131; a color videosignal output 132; and a terminal 133 which is arranged to receive asignal relative to the rotation phase of the above-stated rotary heads.Assuming that the video signal is an NTSC signal, the level of the inputsignal received by the terminal 133 is a rectangular wave of 30 Hz thelevel of which alternately becomes high and low at every 1/60 sec. Thecircuit arrangement further includes a head switching signal formingcircuit 134; a differentiation circuit 135; a one-shot multivibrator136; an OR gate 137; an audio signal reproduction amplifier 138; and anoutput terminal 139 for a reproduced audio signal. The embodimentoperates as follows:

Reproduced signals obtained from the heads 120 and 121 are respectivelysupplied, via amplifiers 122 and 123, to the head change-over switch124. The head change-over switch 124 is arranged to be controlled by ahead switching pulse signal formed by the head switching signal formingcircuit 134 according to the rectangular wave signal of 30 Hz suppliedto the terminal 133. A reproduced mixed signal which is made into acontinuous signal by the head switch 124 is supplied to the LPF 125, HPF126 and BPF 127. A low zone converted chrominance signal is separated bythe LPF 125 and is processed by the chrominance signal processing system128 in a known manner and is thus brought back to its original band(which is 3.58 MHz in the case of an NTSC signal). The chrominancesignal which is thus processed is supplied to the mixer 130. A frequencymodulated luminance signal is separated by the HPF 126 and undergoesknown processes, such as AGC, frequency demodulation clipping, clamping,etc., which are carried out in the luminance signal processing circuit129. The luminance signal thus processed is then supplied to the mixer130. A reproduced color video signal, which is thus obtained from themixer 130, is produced from the terminal 132 via the reproductionamplifier 131. Meanwhile, the frequency modulation audio signalseparated by the BPF 127 is demodulated by the frequency demodulator 3via the limiter 29. The demodulated audio signal is supplied to theterminal A of the switch 52 via the LPF 4 and the delay circuit 50, andalso to the other terminal B of the switch 52 via the LPF 51. In thesame manner as in the embodiment shown in FIG. 10, the signal producedfrom the switch 52 is thus obtained through the LPF 51 which is of lowcut-off frequency during the period of drop-out generation and thetransient noise generation.

The mixed signal is made into a continuous signal by the head switch 124as mentioned in the foregoing. However, it is inevitable that some smalltime discrepancy arises at each joint of the continuous signal. In thecase of a video signal, a discontinued part due to such discrepancy canbe arranged not to appear in the reproduced picture. Whereas, in thecase of an audio signal, occurrence of a discontinuance due to headswitching during the period of 1/60 sec. results in a noise in thereproduced signal. In the event of an audio signal recorded withfrequency modulation, in particular, the reproduced frequency modulatedsignal becomes discontinuous. If the frequency modulated signalincluding such a discontinued part is demodulated, a noise of largeamplitude and a transient noise appear in the demodulated audio signal.The noise is generated at the time of switch-over from one head to theother and also during the ensuing operation period of the phase lockedloop included in the frequency demodulator 3. In reproducing a frequencymodulated audio signal, therefore, the same process that is performed atthe time of occurrence of a drop-out must be carried out also at thetime of head switching.

In the head switching signal forming circuit 134 of FIG. 12, a headswitching signal is generated by suitably adjusting the phase of theabove-stated rectangular wave signal of 30 Hz and the phase adjustedsignal is supplied to the head change-over switch 124. This headswitching signal is supplied also to the differentiation circuit 135.The differentiation circuit 135 is arranged to produce a hair-like pulseat the timing for head switch-over either in a positive or negativedirection. This hair-like pulse triggers the one-shot multivibrator 136to cause the vibrator 136 to produce a signal which momentarily becomesa high level at the time of head switching. A logical sum is obtained atthe OR gate 37 from this signal and a drop-out detection signal producedfrom the drop-out detection circuit 5. Then, a noise removing process isperformed at the time of head switching in the same manner as at thetime of occurrence of a drop-out. In other words, the demodulated audiosignal is arranged to be produced from the switch 52 via the LPF 51which is of low cut-off frequency at the time of head switching and atransient noise generating period which follows the head switchingprocess. The audio signal thus obtained from the switch 52 is producedvia the reproduction amplifier 38 from the terminal 39. The advantage ofthe VTR which is provided with the above-stated reproduction system isas follows: During the periods of occurrence of discontinued parts inthe reproduced frequency modulated audio signal due to the occurrence ofa drop-out and head switching and ensuing transient noise generatingperiods, the reproduced audio signal is replaced with a low frequencyreproduced audio signal which is free from noises of large amplitude andtransient noises, so that a satisfactory reproduced audio signal can beobtained.

FIG. 13 shows a frequency modulated audio signal reproducing system of aVTR arranged as a further embodiment of the invention. FIG. 14 is atiming chart showing output waveforms at point (a)-(g) shown in FIG. 13.

The embodiment includes a reproducing head 10 which is arranged toreproduce a frequency modulated audio signal. The reproduced frequencymodulated signal is supplied to the a limiter 29 which forms a signalcorresponding to frequency modulation by removing the amplitudecomponent of the reproduced signal. The signal from the limiter 29 issupplied to a phase comparator 30 of an APC oscillator 66 which includesa phase lock loop (PLL). The phase comparator 30 produces an errorsignal. The error signal is supplied to a voltage controlled oscillator(VCO) 31 through one terminal B of a switch 68, an LPF 70 and oneterminal B of another switch 72. The phase comparator 30 compares theoutput of the VCO 31 with the above-stated signal corresponding to thefrequency modulation and is arranged to form a so-called PLL. Theoscillation output of the VCO 31 is supplied to a frequency demodulator3. The demodulator 3 then produces a reproduced audio signal to anoutput terminal 9.

If a drop-out is caused to occur in the reproduced frequency modulatedaudio signal by, for example, dust or the like stricking to therecording medium, the waveform of the output of the limiter 29 becomesas shown in FIG. 14(a). If the output is left in this state, a noise oflarge amplitude would occur in the reproduced audio signal. In the eventof occurrence of such a drop-out, a drop-out detection circuit 6produces a pulse as shown in FIG. 14(b). This pulse shifts theconnecting position of the switch 68 to the terminal A. In this manner,during a drop-out generation period, an error voltage produced from thephase comparator 30 is held by an error holding circuit 60 at a valueobtained immediately before the occurrence of the drop-out. Accordingly,the VCO 31 comes to hold the oscillation frequency obtained immediatelybefore the drop-out. Further, in synchronism with the fall of thedrop-out detection pulse (b), a monostable multivibrator 7 is turned onfor a predetermined period of time as shown in FIG. 14(c). This signaland the drop-out detection pulse signal are supplied to an OR gate 8,which then produces a switching pulse as shown in FIG. 14(d). In otherwords, during the drop-out period, the VCO 31 is holding the frequencyof the frequency modulated audio signal obtained immediately before thedrop-out. Further, during the period in which the high level signal isproduced from the monostable multivibrator 7, a disorder within theabove-stated PLL is compensated for by phase locking the output of theVCO 31, which is kept at the predetermined frequency immediately afterthe drop-out, and the reproduced frequency modulated audio signal whichis appearing again. With the LPF 62, which is of a lower time constantthan the LPF 70, connected to the error holding circuit 60 during thisperiod, abnormal oscillation can be prevented.

The error voltage produced from the phase comparator 30 is as shown inFIG. 14(e). This voltage has a signal waveform centering a predeterminedreference voltage when there is no drop-out. In the event of a drop-out,this voltage is kept at a predetermined level during the drop-outperiod. Immediately after the end of the drop-out, the voltage comes tobe corresponding to a phase difference between the reappearingreproduced frequency modulated audio signal and the output of the VCO31. The disorder which takes place at this point of time is graduallycompensated by the LPF 62 which is of a larger time constant and thecondition before occurrence of the drop-out is regained at the fall ofthe output of the monostable multivibrator 7.

With the reproduction system of the VTR arranged as described above, theoutput of the phase comparator 30 included in the PLL is kept at apredetermined level during the period of occurrence of a drop-out.Therefore, compared with the arrangement to pre-hold the reproducedsignal after frequency demodulation, the abnormal oscillation of the VCO31 of the PLL at the end of the drop-out period, i.e. the error voltageof the phase comparator 30 does not become high, so that the transientnoise generation period can be shortened. Further, in the case of thisembodiment, a reproduced signal which is relatively close to theoriginal signal can be obtained by making the response speed of the PLLslower with the time constant of the filter arranged to be large duringa period including the time immediately after the end of the drop-out.

FIG. 15 shows a frequency modulated audio signal reproducing system of aVTR as a still further embodiment of the invention. In FIG. 15, thecomponents similar to those shown in FIG. 13 are indicated by the samereference numerals as those used in FIG. 13 and are omitted fromdescription here. In the case of this embodiment, the output of an APCoscillator 66 (the output of a VCO 31) is used only while the output ofan OR gate 8 is at a high level. Meanwhile, during a period irrelativeto the drop-out, the output of a limiter 29 is allowed to be demodulatedby a frequency demodulator 3. The switch-over between these outputs areeffected by controlling a switch 82 with the output of the OR gate 8.The embodiment includes a delay circuit 80 which is arranged tocompensate for a time delay of the output signal of the APC oscillator66 due to the LPF 62. The same advantageous effect as the arrangement ofFIG. 13 can be obtained by the arrangement shown in FIG. 15.

Further, in the embodiments shown in FIGS. 13 and 15, the output of theVCO 31 is used as the output of the APC oscillator 66. However, sincethe output of the switch 72 which is an input signal for the VCO 31 isconsidered to approximately coincide with the demodulated audio signal,it is possible to use the output of the switch 72, i.e. the output ofthe LPF 62 or LPF 70, as the demodulation audio signal as it is.

While in the embodiments given, the invention is described as applied toreproduction of a frequency modulated audio signal, the invention can beadvantageously applied to a device arranged to reproduce a signalrecorded by frequency modulating an information signal having no timecorrelativity.

What we claim:
 1. A reproducing device for a frequency modulated signal,comprising:(a) reproducing means for reproducing a frequency modulatedsignal from a recording medium; (b) forming means for forming a signalvariable in level in response to variation in frequency of the frequencymodulated signal reproduced by said reproducing means; (c) generatingmeans for generating a pilot signal relative to a discontinued part ofthe frequency modulated signal reproduced by said reproducing means; (d)removing means for removing a high frequency zone of the signal formedby said forming means; and (e) control means for controlling, inaccordance with said pilot signal, the frequency zone which is to beremoved by said removing means.
 2. A device according to claim 1,wherein said forming means includes frequency demodulating means forfrequency demodulating the frequency modulated signal reproduced by saidreproducing means.
 3. A device according to claim 2, wherein saidremoving means includes a first low-pass filter and a second low-passfilter which is of a lower cut-off frequency then said first low-passfilter, said first and second low-pass filters being arranged to filtera signal generated by said frequency demodulating means, respectively.4. A device according to claim 3, wherein said control means includes aswitching circuit arranged to selectively produce a signal comingthrough said first low-pass filter, and a signal coming through saidsecond low-pass filter.
 5. A device according to claim 1, wherein saidforming means includes a controllable oscillator, comparison means forphase-comparing a signal generated by said controllable oscillator withthe frequency modulated signal reproduced by said reproducing means, andfrequency demodulating means for frequency demodulating a signalgenerated by said controllable oscillator, said controllable oscillatorhaving an oscillation frequency variable in response to a signal outputby said comparison means.
 6. A device according to claim 5, wherein saidremoving means includes a low-pass filter for filtering the signaloutput from said comparison means to supply said controllableoscillator, and said control means includes a time constant varyingcircuit which is arranged to vary the cut-off frequency of said low-passfilter in accordance with said pilot signal.
 7. A device according toclaim 5, wherein said removing means includes a first low-pass filterand a second low-pass filter which is of a lower cut-off frequency thansaid first low-pass filter, said first and second low-pass filters beingarranged to filter the signal output by said comparison means,respectively.
 8. A device according to claim 7, wherein said controlmeans includes a switching circuit arranged to selectively supply asignal coming through said first low-pass filter and a signal comingthrough said second low-pass filter said controllable oscillator.